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bq77PL900 UV and UVH limit and test by 17523
The paragraph before table 12 in the datasheet SLUS844B reads as follows:
" When the undervoltage threshold and the hysteresis values are high, then undervoltage recovery may not occur. To avoid this, Table 12 should be used for assistance in configuration. "
What does this mean and how can I test to understand this behavior?
While the exact intent of the datasheet author may not be known, consider the situation where the UV threshold is set at its maximum of 2.9V, UVH is set at its maximum of 1.2V, OV threshold is set at 4.15V, and the part happens to have the maximum +100mV tolerance on UV threshold. When the cells reach UV, the discharge FET is turned off, the charge FET remains on. The pack is then charged. The recovery point for UV would be 2.9 + 1.2 + 0.1 = 4.2V. However at 4.15V without some other mechanism the pack would reach OV threshold and the charge FET would turn off. This would leave the pack with both charge and discharge FETs off and unable to clear either condition, even with perfectly balanced cells. To avoid this situation, the device has an upper limit or plateau of 3.3V for recovery for the combination of UV + UVH as shown in table 12. Even if the settings would put recovery above 3.3V, the device will recover from UV at 3.3V. This limit will avoid the situation where the device is in simultaneous OV and UV in normal conditions, although this can still occur with significantly unbalanced cells.
To quickly test the device behavior regarding undervoltage recovery, a user might connect a board with simulated cell voltages using resistors in the range of 10 - 1k ohm and one or more isolated adjustable power supplies as desired. As one or more cell voltages are lowered below the undervoltage threshold the DSG FET should be observed to turn off using test points and/or the status register. As the voltage is increased so all cell voltages are above the UV + UVH level, the discharge FET should recover. With different settings of the UV and UVH, the user should observe that the recovery is not above 3.3V with tolerance, even though the sum of the settings is above 3.3V. Remember that the UV has a programmable delay which may be disabled with the ZEDE function for faster test.
While this method provides a quick test to help understand and observe the maximum limit described in table 12, the user is responsible for implementing design verification and production tests to be sure their system functions as intended.
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